By Olesya Dmitracova

LONDON (Reuters) - A gargantuan lawsuit between two of post-Soviet Russia's richest and most powerful men reached its climax in a London courtroom Tuesday, with lawyers for tycoon Boris Berezovsky making their case that he was extorted into turning the crown jewel of his business empire over to billionaire rival Roman Abramovich.

The $6 billion case has thrown a spotlight on the shady business dealings of post-Soviet Russia, when opaque privatization deals turned a handful of insiders into the owners of multi-billion dollar natural resources firms.

It has also captivated the legal industry in Britain, whose globally respected, tradition-bound courts - where lawyers still wear powdered wigs - have become the venue of choice for rich Russians to sue each other, generating massive fees.

Berezovsky, 65, accuses Abramovich - known in Britain as the owner of Chelsea soccer club - of intimidating him into selling his stake in oil firm Sibneft at a knockdown price. Abramovich, 45, denies Berezovsky ever had an interest in Sibneft.

Speaking in a courtroom packed with bodyguards and ranks of lawyers and aides Tuesday, Berezovsky's lawyer devoted a large part of his closing statement to examining what he said was untruthful evidence by Abramovich and his witnesses.

"The dishonesty of Mr Abramovich and his key witnesses, their cynical manipulation of evidence and indeed of the trial process, is...perhaps the most important of the general points which my Lady will wish to have in mind when weighing up the evidence and making findings of fact in this case," Laurence Rabinowitz told judge Elizabeth Gloster.

A Kremlin insider in the 1990s under former President Boris Yeltsin, Berezovsky left Russia after falling out with Yeltsin's hand-picked successor Vladimir Putin. He says he gave up his Sibneft stake because he feared that if he refused, Abramovich would ensure Putin had the shares expropriated.

Abramovich says he paid Berezovsky $2 billion for his political patronage and protection from criminal gangs, but not as dividends from Sibneft because Berezovsky was never an owner.

Abramovich has since sold Sibneft to the Russian state natural gas monopoly Gazprom.

CLUES INTO PUTIN'S RUSSIA

The trial has been tabloid fodder in Britain ever since a tussle between the two tycoons and their retinues of bodyguards in a Hermes luxury boutique in London, when Berezovsky spotted Abramovich and served him with a writ.

During Tuesday's hearing, Berezovsky appeared relaxed, often laughing and conferring with his younger girlfriend. Abramovich, sitting at the opposite end of the courtroom, listened intently to the Russian translation of the proceedings in headphones.

The trial, which started in early October, is being followed closely by Russia watchers from London and Moscow for new clues into Russian business and politics under Putin, now prime minister but expected to become president again this year.

Abramovich and Berezovsky were close allies when making their fortunes in Russia in the 1990s under Yeltsin.

Since then, Berezovsky has become a sworn enemy of Putin, fending off requests to extradite him from London on Russian criminal charges by arguing that he could not get a fair trial in Russia. Abramovich became a Putin ally and prospered.

Britain is home to a string of high-profile Kremlin critics, one of whom, Alexander Litvinenko, was murdered in London in 2006. Moscow's refusal to extradite the man London suspects in his killing remains the largest single source of strain in Russian-British relations.